It is known that, in the petroleum industry, during the past fifty years, catalytic cracking of hydrocarbon feedstocks has progressively replaced thermal cracking. The fixed catalyst beds initially used have been rapidly replaced by mobile beds, particularly by fluidized beds, which led to the process currently known as fluidized-bed catalytic cracking or fluid catalytic cracking (FCC process).
In such processes, the cracking of the feedstock is carried out at a temperature of about 500.degree. C. at a pressure close to atmospheric pressure and in the absence of hydrogen. During cracking, the catalyst becomes covered with coke and heavy hydrocarbons and is continuously regenerated outside the cracking reactor. The heat generated during regeneration by the combustion of the coke and of traces of residual hydrocarbons in the presence of air or oxygen brings the catalyst particles to the desired temperature, said particles then being recycled to the reactor.
Various types of catalysts can be used, and in this respect the reader is referred to, for example, U.S. patent Ser. No. 4,724,067 issued Feb. 9, 1988 to F. Raatz et al. or its European counterpart [EP]-A-0 206 871. Both patents are hereby incorporated herein by reference.
Said FCC processes produce automobile gasolines of much better quality and in much higher yields than do thermal cracking processes.
Said processes are usually carried out with upflowing catalyst particles, but this leads to a number of drawbacks because the gases present have a tendency to rise whereas the catalyst particles, because of their weight, resist ascending movement. As a result, in current reactors the C/O ratio of the catalyst flow rate "C" to the flow rate "O" of the feedstock to be treated is generally from 3 to 7 and usually close to 5.
More precisely, in an upflow reactor, the catalyst particles have a tendency to redescend, and the catalyst bed is supported and entrained by the vaporized feedstock to be cracked and by the lift gas. Hence, the catalyst flow rate "C" cannot be increased at will without risking an excessive slow-down in the rise of the catalyst particles. Downflow reactors obviously do not pose this problem.
These limitations of prior-art upflow reactors (also known as risers) manifest themselves particularly in the cracking of feedstocks with a high content of basic nitrogen compounds. The basic nitrogen compounds present in such feedstocks include, in particular, pyridine, quinoline, acridine, phenanthridine, hydroxyquinoline, hydroxypyridine and the alkyl derivatives thereof. With such feedstocks, the drop in conversion may be up to 15% compared to a normal feedstock. It is known, in fact, that basic nitrogen attaches itself to the active sites of the catalyst thus altering its catalytic properties.
Moreover, in upflow reactors, particles accumulate in the vicinity of the reactor walls which causes hydrocarbon overcracking in these areas. This gives rise to the formation of coke and hydrogen in place of the desired high-octane-number products and leads to insufficient feedstock conversion in the center of the reactor where fewer particles are present.
Moreover, although the catalyst particles, overall, rise in the reactor, some of them can locally redescend. This phenomenon, known as back-mixing, also results in a local drop in conversion, because the redescending particles are partially deactivated and exert a lesser effect on the feedstock than do the rising particles. This phenomenon is the more troublesome the lower the aforesaid C/O ratio.
To eliminate said drawbacks which make the catalytic cracking of feedstocks with a high basic nitrogen content very difficult and uneconomical, it has been proposed to subject the feedstocks to hydrotreatment which results in a reduction of the basic nitrogen content, but which requires high pressures and temperatures and, hence, is expensive.
To eliminate the basic compounds, it has also been proposed to use solid absorbents or solvents that are not miscible with the feedstock. Such a process, however, is long and costly.
The same is true for feedstock treatments with acid additives to neutralize the basic nitrogen compounds. For this reason, catalysts suitable for FCC processes and resistant to basic nitrogen have preferably been used (see "Nitrogen Resistance of FCC Catalysts" by J. Scherzer and D. P. McArthur, paper presented at "Katalistiks 7th Annual Cat Cracking Symposium", Venice, Italy, May 12-13, 1986), hereby incorporated herein by reference.